Prestressed drill stem



Nov. 8, 1960 E.A.`GU| DENzoP|-| ETAL 9 2,959,193

PRESTRESSED DRILL STEM Filed Feb, 4. 1957 Il!! Utlrla 1 2,959,193 Patented Nov. 8, 1960 fr a 1C@ PRESTRESSED DRILL STEM Edwin A. Guldenzoph, Saginaw, and Irving S. Houvener,

Midland, Mich., assignors to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Feb. 4, 1957, Ser. No. 637,935

19 Claims. (Cl. 1358-25) This invention relates to a prestressed drill stem or the like. A drill stern embodying the invention is desirably made in separable sections, being in accord with conventional practice in this regard.

There is a problem of long standing solved by this invention. Wells are being drilled to such a depth that the weight of the drill stem may exceed the tensile strength of the material of which the stem is made. Use of metal of increased yield point would enable the length of the drill stem to be increased without increase in weight but is objectionable from the standpoint of expense.

I have considered the possibility of re-enforcing a drill stem by the addition of metal of higher yield point. However, this does not solve the problem because the yield point of the composite structure is essentially the same yield point as the yield point of the weakest component part thereof.

The present invention contemplates a composite structure in which the low yield point component is under compression and the high yield point component is under tension. As the drill stem is loaded, the first increments of weight decrease the compression of the low yield point member and increase tension on the high yield point member until the compression on the low yield point member reaches O and continued loading places the low yield point member in tension until, finally, both members approach their yield point concurrently. By theseV means, the full yield strength of the high yield point member may be utilized, with the result that each prestressed drill stern section will sustain a load ranging from about 9 to about 17 percent increase over the tensile strength of an otherwise identical unstressed drill stem section. By using prestressed sections according to the present invention throughout the upper end of a drill stem in an unusually deep well, it is possible to operate safely at much greater depths than would otherwise be attained.

The preferred structure is one in which the torsional stress is communicated in the usual manner through a tubular drill stem section of low yield point, several of the tubular sections being screw threaded together in accordance with conventional practice. However, in accordance with the present invention, each such low yield point tubular section is provided internally with an,

anchorage adjacent the `coupling heads at its respective ends and a rod iixed in one such anchorage is provided with a nut tightened against:` the other to prestress the rod under tension and the portion of the tube between said anchorages being under compression, the rod being of high tensile strength material off much smaller cross section than the tube in order to minimize the amount of high tensile strength material used in the device.

In the drawings: n

Fig. l is a view in side elevation on a small scale of a number of coupled drill stem sections.

Fig. 2 is a view in axial section on a larger scale through portions of two drill stem sections embodying the invention.

Fig. 3 is a further enlarged detail view in perspective showing in relatively separated positions one of the tension rod anchorages including the nut by which the parts are prestressed.

Fig. 4 is a fragmentary detail view `on a scale somewhat smaller than Fig. 2 showing a modied embodiment of the invention.

The drill stem is made up of a number of sections 5 each having terminal coupling heads 6 and 7. The coupling head 6 has a tapered counterbore at 8 with female threads at 9. The head 7 has a correspondingly tapered terminal portion 10. Fig. l shows the male coupling 10 of the upper drill stem section 5 threaded into the female threads 9 in the coupling head 6 of the next lower drill stem section.

For purposes of the present invention each drill stem section is desirably provided in its respective heads with anchorages for a tension rod made of metal having a very materially higher yield point than that of the metal of which the tube 5 is made. The rod 15 illustrated in Fig. 2 is solid, whereas the corresponding rod 150 in Fig. 4 is tubular, like the stem. Rod 15 has threaded ends at 16 and 17, each of which is in threaded connection with a shouldered nut 20` having a sleeve portion 21 of reduced radius fitting into a sectional seat which comprises segments 22 and 23. The particular form of the seat is immaterial, as long as it can be introduced successfully into the head 6 or 7 toengage a shoulder at 24 which, in practice, is made by undercutting a groove 25 in the head. In practice the seat segments can be held temporarily in the groove through the use of relatively heavy grease. When the nut 20 has its extension sleeve 21 introduced into the seat, it holds the seat segments securely in the undercut groove 25 to provide a solid anchorage abutted by the shoulder 26 of the nut.

For prestress purposes, either nut may be treated as a xed anchorage and the other rotated to subject the rod 15 to tension and the tubular drill stem 5 to compression. In referring to prestress of the tubular stem, I am particularly interested, of course, in the relatively small diametered portion intermediate the coupling heads. Each nut is provided with relativly large openings 28 which not only accommodate fluid ilow through the whole length of the assembled drill stem, in cases in which such iiow is desired, but also are adapted to receive the prongs of a Spanner wrench or other tool used to tighten the nut.

The resulting drill stem section is handled as a unit and can be assembled with other like sections in the manner indicated in Fig. 1. Conventional drill stem sections may be used whenever their tensile strength is adequate, it being intended that the prestressed sections herein disclosed will ordinarily be used only at the upper end of an unusually long drill stem, the weight of which might cause breakage but 4for the use of these special sections to make up the heavily loaded part thereof.

In the construction shown in Fig. 4 the tubular drill stem or pipe 5 remains unchanged but its terminal heads 60 and 70 have been altered slightly from those previously disclosed. Terminal head 70 has a tapered form of undercut groove 250 within which a Slotted end 29 of the tubular rod is expanded by the tubular wedge 30, thus securely anchoring the rod to head portion 70. At its other end, the tubular rod 150 is provided with internal screw threads at 31 with which the external threads of a tubular draw nut 200 are engaged. The shoulder 260 of the ldraw nut engages a shoulder 240 in the head 60 of the drill stem, the shoulder being for-med by a counterbore at 32 which is sufficiently large to receive the nut 200. Notches 33 in the outer end of the nut receive a wrench 3 by which the nut may be tightened in the tubular rod 150 to prestress the rod and pipe in the manner already described. In this construction the inner bore 280 of the nut provides wide-open communication between each drill stem section 150 and the next.

The underlying principles are as follows:

The modulus of elasticity is practically the same (Z9-30x106 psi.) yfor all steels and thus they will all stretch approximately to the same degree under a given load. Consequently a high yield point steel does not differ materially in its modulus of` elasticity from a low yield point steel. The significant diiference lies in the fact that the high yield point steel will stretch farther before reaching its yield point.

Without prestressing, the high yield point of a reinforcing member can never be fully utilized. Withoutprestress, the rod 15 and the tube 5 would stretch substantially in unison until the yield point of the tubular stem section is reached. At that point the tube S would yield and the entire load would be imposed on the rod 15 which, naturally, would immediately yield also, being wholly inadequate to sustain the entire load.

The effect of prestress can best be understood if it is regarded as relieving the low yield point member of ten sion until the other member has been sufficiently tensioned so that both members will yreach their yield points substantially concurrently. Prestress accomplishes this result as will be apparent from the `following, examples. Let it be assumed that the yield point of the rod is reached at 200,000 p.s.i. while the yield point of each tube is reached at 80,000 p.s.i. The gures given are based on dimensions yand materials which would be appropriate for the purpose, the drill stem tube being 41/2" OD. and 3.64 LD. with a cross sectional area of 5.5 square inches and made of heat treated alloy steel with a weight (with couplings) of 22 pounds to the foot. The high yield strength steel rod is l in diameter and has anarea in cross section of .785 `Square inch.

In the following formulas, the symbols have the following meanings:

YR-Yield point of rod member YT--Yield point of tube member ARCross sectional area of rod member AT--Cross sectional area of tube member #-Pounds The proper prestress of the rod member is determined from the following formula:

In the foregoing example, the proper prestress is:

@umg-naamw@ i The compressed load which must be applied to the assembled rod and tube to give the desired prestress is represented by the following formula:

The total allowable load is represented by the following formula:

[ YR TX AR]+ (AMAT) YT ameter) Le., a ratio of areas of 7:1, the total cross sectional metal area is 6.285 square inches and the total allowable load is:

The following table shows what happens when the prestressed section is subjected to various loads:

It will be noted from the foregoing table that the assumed maximum stress'on the high yield point rod and the maximum permissible stress on the low yield point tube are reached simultaneously under a load of 549,900 lbs. Without the prestress the tube would have broken at 502,800 lbs. The percentage gain in strength by reason of prestress is 9.35 percent. With double the area of cross section of the rod the percentage increase attributable to prestress would be 16.7 percent.

Calculation shows that the shear stress resulting from torque in the rotation of the drill stem has negligible effect on the total stress and can be disregarded in the foregoing calculations.

Assuming that the upper lengths of drill stem sections are provided with a high yield point rod (e.g., 200,000 p.s.i., yield point) having a cross sectional area of 1.5 square inches and prestressed under a tension of 60,000 lbs. to the square inch, the net gain in load supporting capacity of the drill stem is 210,000 lbs. and 7,750 feet of additional drill stem can be supported.

As shown in the drawing it will be understood that the cross section of the lower yield point member at'the anchorage portions, or coupling heads, must be sufiiciently great to carry the increase in load resulting from the combination of the prestressed members. The conventional externally upset drill pipe, the ends of which have a greater cross sectional area of metal than the pi e intermediate the ends, is generally suitable for the lower yield point member.

We claim:

il. A drill stem comprising prestressed members connected at axially spaced points, one of said members comprising a material of relatively low yield point under compression and the other of said members comprising a material of relatively high yield point under tension whereby, under load, the full strength of the member of high yield point material is utilized to increase the weight which will be sustained before breakage.

2. In a device of the character described, the combination with a tubular member of low yield point metal and a rod of high yield point metal within said tubular member, the rod and tube being connected at axially spaced points under prestress tensioning the rod and compressing the tube, the yield point of the tubular meme ber under tension of both members being thereby adjusted toward the yield point of the rod.

3. The device of claim 2 in which the connection between the rod and tube provides for uid communication through the tube.

4. The device of claim 3 in which the communication is through the rod, the rod also being tubular.

5. A drill stem section adapted to be coupled to other like sections to constitute an elongated drill stem, the said section comprising a tube of relatively low yield point material provided at its ends with threaded couplings, a rod disposed within the tube and comprising relatively high yield point material, the tube having an anchorage for said rod adjacent one of said couplings and provided adjacent its other said coupling with a seat, a nut engaging the seat and in threaded connection with the rod, the rod and tube being prestressed with the rod under tension and the tube under compression.

6. The drill stem section of claim 5 in which the rod is tubular.

7. The drill stem section of claim 5 in which the tube anchorage rst mentioned comprises an. annular wedge,

the tube having an undercut recess and the rod comprising a tube in which the wedge is disposed, the tube being expanded by the wedge into the recess.

8. The device of claim 7 in which the tube has a counterbore and a shoulder, the nut being receivable into the counterbore and engaged with the shoulder, the shoulder providing said seat.

9. A drill stem section adapted to be coupled to other like sections to comprise an elongated drill stem, said section including a tube member having a rod `member inside of it, the tube member comprising relatively low yield point steel and the rod member relatively high yield point steel, the tube member having couplings at its ends and provided adjacent one of said couplings with a rod anchorage to which the rod member is attached, an adjustable anchorage connecting the other ends of said members, the adjustable anchorage having screw threads and being adjusted to a position prestressing both members with the rod member under tension and the tube member under compression.

10. The drill stern section described in claim 9 in which the adjustable anchorage comprises ring segments disposed upon a seat which one of said members is grooved to provide, and a nut in screw threaded connection with the other member and having a shoulder in pressure engagement with the ring segments, said nut having a portion in peripheral engagement with the ring segments for the positioning thereof in the groove.

11. 'I'he device of claim 9 in which the tube member is provided with an internal groove, the adjustable anchorage comprising segments receivable into an end of the tube member and positioned as a ring within the groove, the adjustable anchorage further comprising a nut having a shoulder seated on said segments and a portion of reduced diameter extending into such segments in engagement with the inner periphery of the ring to maintain the segments in the groove.

12. The device of claim 9 in which both of said anchorages have openings for accommodating lluid ow through said drill stem section.

13. The device of claim 9 in which the rod member is also tubular, the anchorage in said first mentioned tube member comprising an undercut groove and an annular wedge in one end of the rod member, the latter end of the rod member being expanded into the groove, the

tube member being provided at its other end with a counterbore and a shoulder, the adjustable anchorage comprising an annular nut threaded to the tubular rod member and seated on the shoulder, the tubular rod member providing open communication throughout said section.

14. A drill stem section adapted to be coupled to other sections to comprise a drill stem, said section including a tubular member having terminal male and female couplings at its ends and provided at axially spaced points near said couplings with keys disposed on its inner periphery, anchorage means engaging respective keys, and a rod member connected with resepctive anchorage means, one of said anchorage means comprising a nut in threaded connection with the rod member, the tubular member comprising relatively low yield point steel under compression and the rod member comprising relatively high yield point steel under tension developed by the rotation of the nut relative to the rod member.

15. The device of claim 14 in which the respective keys comprise segmental rings for the reception of which the tube member is internally grooved.

16. The device of claim 15 in which both anchorage means comprise spanner operable nuts having openings accommodating uid flow through the drill stem section.

17. A drill stem section comprising a tube terminally provided with male and female couplings and internally having opposed seats, relatively adjustable anchorage means supported from respective seats, and a rod connecting said anchorage means under tension and subjecting said tube to compression between said seats, the tube comprising relatively low yield point steel and the rod comprising relatively high yield point steel, the prestress of the rod and tube being such that the yield points of the tube and rod at least substantially coincide.

18. The device of claim 17 in which the anchorage means comprise nuts in threaded connection with the rod for developing said prestress, each of said nuts being provided with through openings for permitting fluid flow through said section.

19. The device of claim 17 in which the rod is tubular and has one end xed by one of said anchorage means to said tube, the other anchorage means comprising a nut in threaded connection with the rod and engaged with a seat with which the tube is provided, the tubular rod providing free communication from end to end of said section.

References Cited in the file of this patent UNITED STATES PATENTS 2,453,079 Rossman Nov. 2, 1948 FOREIGN PATENTS 187,487 Germany July 26, 1907 

